The present invention relates to an agitator drive system for use in an automatic washer including an alternately reversing drive motor, especially a permanent split capacitor.
In U.S. Pat. No. 4,779,431, a drive system for an automatic washer is disclosed that has a high slip motor connected to a drive reduction mechanism for driving an energy absorbing agitator. The high slip motor is driven with spaced alternating pulses so that the agitator accelerates in one direction and then slows after the end of the pulse followed by an acceleration and then slowing in a second direction so that sinusoidal agitation is produced by a pulse input to the motor. It was found that sinusoidal motion in a washing machine produced improved washing characteristics, with reduced fabric damage. Such agitation motion was previously available only by using relatively large, complex and expensive reversing transmissions.
In U.S. Pat. No. 4,542,633, an agitating drive washing machine is disclosed having an agitating wheel connected to a reversible drive motor by means of a pulley arrangement. The agitating wheel is shown as having short, thick blades. A rotational angle detector is connected to a control that causes the motor, which is described as having rigidity, to rotate the agitating wheel about a predetermined angle. Upon reaching the predetermined angle, the motor is not energized until the agitator wheel has stopped, at which time the motor is operated in a reverse direction to again rotate the agitator wheel about the predetermined angle.
An automatic washer manufactured by an assignee of U.S. Pat. No. 4,542,633, and appearing to incorporate the teachings of that patent, has been observed to have an agitator stroke angle of approximately 360.degree. for each stroke at a rate of 70 strokes per minute. A high rigidity motor causes a nearly constant agitator speed in each rotational direction following a brief, rapid acceleration, and followed by the coasting to stop action.
Thus, automatic washer drive systems having reversing motors for producing a generally square wave agitation curve are known. An agitation curve is defined herein as the rotational speed of the agitator over time. A square wave agitation curve is thus a constant speed in one direction followed by a constant speed in the opposite rotational direction. Correspondingly, a sinusoidal agitation curve would result from a gradual acceleration followed immediately by a gradual deceleration of the agitator in each rotational direction.
In U.S. Pat. No. 3,315,500 to Brundage et al., a mechanism is provided as a drive system for an agitator using a motor operating at a constant speed in one direction to drive a hydraulic transmission which provides very rapid reversal of the agitator through a reversing valve and, thus, is a square wave type agitation motor. An elastic coupling is used between the agitator and the agitator drive shaft to reduce the shock which would otherwise be delivered to the agitator at each reversal.
An automatic washer having an axial air gap induction motor is disclosed in U.S. Pat. No. 4,232,536. The motor is repeatedly reversed to oscillate a vertical axis agitator through a planetary drive coupled therebetween, the motor being operated at high speeds. The desirability of providing a soft starting action of the agitator each time it reverses is recognized, the soft starting action being provided by the backlash associated with the planetary drive mechanism.
The appliances disclosed in the above-cited references demonstrate the importance of the type of motor used. The motors employed greatly influence the quality of operation of these appliances.
A properly designed permanent split capacitor (PSC) motor has many characteristics which make it ideal for a continuously reversing application, such as in an automatic washer. It does not require a starting switch, full load efficiency is 70%, the ratio of starting to breakdown torque is about 0.6, electronics could be added to extend performance capability, and it is inherently shorter in axial length than other motor types. Its reliability and many years of existence makes this a very low risk motor for this application.
Permanent split capacitor motors and the design thereof are well known in the prior art. Typically, the run and auxiliary windings are wound in space pole quadrature, i.e., 90 electrical degrees with respect to each other; the former is usually wound first, and then the latter is wound on top of it with a different number of turns and wire size. The rotor is of the usual squirrel cage construction.
The design parameters of prior art permanent split capacitor motors is well established and any significant deviation from these parameters is considered to result in motors which function poorly. For example, the ratio of the stator bore, D.sub.1, to the outside diameter of the stator is approximately 0.6 for a four pole motor. The well-known empirical formula for establishing a value for this ratio, where P is the number of poles, is: ##EQU1##
Also, for maximum efficiency at the load point, the motor should have a turns ratio in the range 1.5 to 2.5.
FIG. 7A shows the speed-torque curves for a prior art reversing motor in which curve 201 is for one direction of rotation and curve 203 is for an opposite direction of rotation. The load line 205 intersects both curves 201 and 203 as shown. If the motor winding is reversed at point 207, the system will follow curve 205 to point 209 where it will stabilize instead of reversing. The present invention overcomes this drawback in the prior art. Instead, the curves 201 and 203 are a function of the turns ratio of the windings.
An excellent summary of the design and operational parameters of prior art permanent split capacitor motors is found in "Theory and Design of Small Induction Motors" by Cyril G. Veinott, published by McGraw Hill Book Company, Inc. in 1959.
In a continuously reversing application of the permanent split capacitor motor, it is necessary to greatly increase the size of the motor to achieve rapid acceleration under load. However, it has been found that large increases in physical motor size produce only small increases in motor performance. This is because most of the increase in torque is used to turn the rotor which has increased inertia. Again, the present invention overcomes this drawback in the prior art.